The present invention relates to a power control device of an electric tool, a method for power control of an electric tool, and an electric tool provided with a power control device.
Power control devices of the above mentioned general type are known in the art. They are used for supplying a variable power from an energy source of an electric tool (for example an electric grid connection or an accumulator) to an electric motor of an electric tool. It is known to perform the control of the power by means of a pulse width modulation, or in other words to supply the power of the energy source not permanently, but instead in a pulsed fashion to the electric motor. Depending on the pulse width, the electric motor is set to make available an efficient power: when the pulse width amounts for example to 25% of the period provided by the frequency of the pulse width modulation, only approximately 25% of the maximum possible power is transmitted from the energy source to the electric motor.
The operation of a power control device with a pulse width modulation at a frequency of for example 5 kHz makes it acoustically recognizable by high frequency noise, similar to a whistle. In particular at low rotary speeds of the electric motor, for example during starting of the motor, this noise is significantly hearable and taken by users of the electric tool as disturbing.
From the prior art it is known to increase the frequency of the pulse width modulation, for example to 10 kHz so that the noise is located in a frequency region which is less hearable for the human hearing and can be heard as less disturbing. The increased frequency however leads to the situation that the losses in the transistor producing the pulse of the pulse width modulation, for example a MOSFET, increases. The undesired, additional losses require a transistor with high power data, which leads to a significant cost of the transistor-and thereby of the power control device.